ath9k: add support for reading eeprom from platform data on PCI devices

Some embedded boards store platform data for connected PCIe AR92xx
chips in the system flash instead of a separate EEPROM chip.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>